Linux Kernel Architecture

Chapter 1: Introduction and Overview

next

prev

next

prev

next

prev

next

prev

Figure 1-11: Doubly linked standard list.

struct list_headis called alist elementwhen it is held in a data structure. An element that serves as the
starting point for a list is called alist head.

Pointers that connect head and tail elements of a list tend to clutter up images and

often obstruct the principal intention of a figure, namely, to briefly summarize the

connections of various kernel data structures. I thus usuallyomitthe connection

between list head and list tail in figures. The above list is in the remainder of this

book therefore represented as shown in Figure 1-12. This allows for concentrating

on the essential details without having to waste space for irrelevant list pointers.

Figure 1-12: Simplified illustration of a doubly

linked list. Notice that the connection between

list head and list tail isnotdisplayed, although

it is present in kernel memory.

There are several standard functions for handling and processing lists. We will come across them again
and again in the following chapters (the data type of their arguments isstruct list_head).

❑ list_add(new, head)insertsnewright after the existingheadelement.

❑ list_add_tail(new, head)insertsnewright before the element specified byhead. If the list head

is specified forhead, this causes the new element to be inserted at the end of the list because of

the cyclic nature of the list (this gives the function its name).

❑ list_del(entry)deletes an entry from a list.

❑ list_empty(head)checks if a list is empty, that is, if it does not contain any elements.

❑ list_splice(list, head)combines two lists by inserting the list inlistafter theheadelement

of an existing list.

❑ list_entrymust be used to find a list element; at first glance, its call syntax appears to be quite

complicated:list_entry(ptr, type, member).ptris a pointer to thelist_headinstance of the